In recent years, the science of microwave radiometry has distinguished itself as a part of the general field of environmental remote sensing. Microwave radiometry also has taken on a new name, "passive microwave remote sensing", in contrast to radar, which has come to be known as "active microwave remote sensing." A microwave radiometer is a highly sensitive receiver typically used in radiometry for measuring low levels of microwave radiation. When an object or scene is to be observed by the microwave radiometer, radiation received by the radiometer is partly due to self radiation by the object and partly due to reflective radiation originating from the surroundings of the object. (See Ulaby et al., Microwave Remote Sensing Active and Passive, Volume 1, 1981 Addison-Wesley Publishing Company, Redding, Mass.).
Radiometers fall into two broad classifications: the so-called total power radiometers and the so-called Dicke-type radiometers. For purposes of illustrating the present invention, a total power radiometer is considered. As described by Peckham in "An Optimum Calibration Procedure for Radiometers", International Journal of Remote Sensing, 1989, Volume 10, No. 1, pages 227-236, a total power radiometer consists of a receiver incorporating a band pass filter and a square-law detector and a post-detection integrator. The output signal from the post-detection integrator includes low frequency fluctuations arising from noise at the square-law detector. In microwave radiometers, the effect of these fluctuations on the interpretation of the output signal is very important. Temperature-induced gain and offset changes cause variation in the level of these low frequency fluctuations in the amplified system noise and hence in the square-law detector output. Thus, the radiometer gain and offset need to be periodically calibrated. A typical calibration system periodically switches the receiver input from the object to be observed to one or two calibration targets which provide standard signals with which to interpret the fluctuating output signals.
Recent investigations into passive microwave remote sensing of atmospheric precipitation have suggested that the difference between vertically and horizontally polarized brightness temperatures (brightness temperature is a term used to represent the intensity of radiation) contains useful information about the intensity of precipitation over oceans, and possibly the presence of aspherical particles in the cell tops of cloud formations. The polarization difference can also be used as an aid in detecting snow cover and discrimination between land and water backgrounds. The polarization difference can be detected with a dual linearly-polarized radiometer. In simple terms, this means that the signal received by the radiometer in one plane of polarization contains different information from that received by the radiometer in another plane, and that these "differences" can be used to evaluate the object being observed.
The polarization characteristics of a general radiation field can be completely and uniquely described by mathematics involving four Stokes' parameters, defined in an appropriate basis. The "basis" is a set of axes, or directions in space, along which the components of the electric field vectors of the radiation field lie. The measurement of these polarization parameters for naturally occurring radiation fields has been thoroughly studied within the context of radioastronomy, where the detection and measurement of the circularly polarized components of radiation fields has traditionally been of greatest interest. See, for example, M. H. Cullen, "Radio Astronomy Polarization Measurements", Proc. IRE, 46, pages 172-183, 1958. The reason for this is that many stellar objects emit radiation through nonthermal processes (e.g., synchrotron emission), which typically produce circularly polarized fields.
For the terrestrial troposphere and lower stratosphere, and for frequencies at which nonreciprocal propagation effects caused by the earth's magnetic field can be neglected, only the vertically and horizontally polarized Stokes' parameters (the brightness temperatures T.sub.v and T.sub.h) have been shown to contain significant information. Hence, it is appropriate for instruments observing the earth and its atmosphere to measure, particularly, these two parameters. Because the vertical - horizontal (V-H) polarization basis requires a minimal number (2) of non-zero parameters to convey most of the information in such a radiation field, it is designated as the "natural" polarization basis.
The natural polarization basis is distinguished from the radiometer polarization basis, since the radiometer basis may or may not be aligned with the V- and H-axes. Ideally, the polarization basis of an airborne or spaceborne dual linearly-polarized radiometer should coincide with the natural polarization basis, so that the two radiometer channel outputs will be T.sub.v and T.sub.h (upon proper calibration). Without such coincidence of bases, the two channel outputs will be an undecipherable mixture of T.sub.v and T.sub.h. Unfortunately, the need to image extended regions of the atmosphere by physically scanning the antenna beam makes it difficult to build an instrument which can retain polarization coincidence with the natural basis at every imaged spot. Consider, for example, two common scanning configurations for passive microwave imaging, conical and cross-track scanners. A conical scanner typically consists of a feedhorn and an off-axis parabolic antenna rotating about the feedhorn axis, thus sweeping the antenna beam through a cone-shaped swath. The cross-track scanner consists of a rotating scanning mirror oriented at a 45.degree. angle with respect to the feedhorn axis. The resulting wedge-shaped path of the cross-track scanner typically is oriented transverse to the motion of the host aircraft or spacecraft, yielding a raster scan that is directed below the flight track of the host.
In their simplest configuration, both conical and cross-track scanners employ a feedhorn fixedly secured to the instrument platform. For dual linearly-polarized feedhorns, this results in the feedhorn polarization basis rotating (with respect to the natural basis) during the scan. The angular difference between the two bases is termed the polarization basis skew .phi. (the two bases coincide at .phi.=0). To obtain a zero polarization skew at all scan angles, it has been known in the art to mechanically rotate the feedhorn along with the scanning reflector. However, alignment of the feedhorn and natural polarization bases by mechanical rotation is cumbersome, expensive, and somewhat unreliable, particularly in space.
In an unpublished paper of 1988 entitled "Microwave Precipitation Radiometer", G. S. Parks, T. C. Fraschetti, and D. C. Miller of the Jet Propulsion Laboratory, have suggested the possibility of eliminating the polarization skew throughout the scan of a conical scanner, while using an inexpensive fixed feed scanner, by electronically rotating the polarization basis. Their suggested method was based on the measurement of a cross-correlation between the two orthogonal linearly-polarized radiometer channels. The additional measurement made by this additional "U" channel is denoted "T.sub.fU ". However, the paper made no mention of three major issues concerning the practical implementation of electronic basis rotation. First, and perhaps most importantly, a scheme for calibration was not presented, and without calibration, the information obtained by the U channel cannot be relied upon. Secondly, a simple, practical implementation of the U channel correlator circuitry was not considered. Thirdly, an algorithm for transforming the measured polarization data back into the natural polarization basis was not discussed. In addition, the applicability of electronic polarization rotation to cross-track scanners was not considered.
Accordingly, it can be seen that a need yet remains for a dual-polarized cross-correlating radiometer and a method and apparatus for calibrating the same. It is to the provision of such that the present invention is primarily directed.